1. Field of the Invention
This invention relates to a Hall motor control system in which Hall elements detect the position of a rotor comprising a permanent magnet to switch over driving currents flowing to driving coils.
2. Description of the Prior Art
In the conventional Hall motor control system wherein Hall elements detect the position of a rotor comprising a permanent magnet to switch over driving currents flowing to driving coils, a voltage corresponding to the rotational speed of the rotor or of the motor has been induced in the driving coils and such voltage has been detected by a comparing circuit comprising a switching transistor circuit, whereby when the induced voltage exceeds a predetermined level, the switching transistor circuit is reversed to control the input current to the Hall elements and thus the driving current flowing to the driving coils, thereby imparting a predetermined torque to the motor and controlling the motor to be driven at a predetermined speed at all times.
In the conventional motor control system of this type, however, the voltages proportional to the number of revolutions induced in the coils to which no driving current is then flowing are separated, detected and synthesized by a diode for use as a control voltage, and this has required the use of an element having diode characteristics. In such manner, where the voltage is rectified by the diodes, and the induced voltage is developed, the forward voltage drop V.sub.F in the diode is included as an error in the rotational speed voltage. Such error creating tendency is greater as the number of revolutions is lower and as a result, the ratio thereof to the forward voltage drop in the diode becomes smaller to impair the linearity. Also, the diode is susceptible to temperature variation and therefore, even if compensation therefor is imparted for a certain number of revolutions, the compensation curve will be seriously departed from ideal curve as the number of revolutions is varied and thus, it has been extremely difficult to provide a drive with good temperature characteristic.
Further, in the conventional Hall motor control system of the described type, the detection of the number of revolutions of the motor by the comparing circuit has been done by applying the induced voltage between the base and emitter of the switching transistor to compare the base-emitter voltage V.sub.BE of the switching transistor with the voltage corresponding to the reference rotational speed of the motor, but the induced voltage is produced in superposed relationship with the source voltage and thus, whenever the source voltage is fluctuated for some reason or other, the voltage applied between the base and emitter of the switching transistor is also fluctuated even if the induced voltage is constant, and this has not only failed to obtain accurate comparison but also varied the base-emitter voltage V.sub.BE of the transistor itself with respect to the temperature variation, thus failing to provide an accurate speed control.